Method for decontamination of reactor solutions



May 5, 1959 W. J. MARAMAN ET AL i METHOD FOR DECONTAMINTION OF REACTOR SOLUTIONS Filed April 23,'195'7 METHOD FOR DECONTAMINATION OF REACTOR SOLUTIONS Application April 23, 1957, Serial No. 654,622 6 Claims. (Cl. 23-14.5)

This invention relates to processes for decontaminating I,IJnitecl States PatentI aqueous solutions containing nuclear fission products, and

'particularly to continuous processes for decontaminating aqueous reactor fuels taking the form of a phosphate complex of a ssionable element in a concentrated phosphoric acid solution.

Prioi to the present invention, most of the effort in the field of solution dcpoisoning and decontamination was directed to aqueous solutions of uranyl nitrate, either used in that form as a reactor fuel or prepared from other forms of uranium contaminated with elements of the fis- -sion products category. Prior workers had found that a uranyl nitrate fuel could be decontaminated by a rather complicated process involving the addition of a weak (0.2 M) phosphoric acid and a source of ammonium ions to precipitate uranyl ammonium phosphate. This phosphate was then converted to the intermediate forms of uranous fluoride to uranyl fluoride, which in turn was converted to uranyl oxide, U03. The latter was finally added to nitric acid to form the uranyl nitrate in the reactor fuel, or one of the oxide or fluoride forms was reduced to the metal for fabrication into fuel elements.

Later workers developed the process wherein uranyl nitrate is extracted from its aqueous solution by organic compounds such as tributyl phosphate, diethyl ether, methyl isobutyl ketone, amyl alcohol, or amyl acetate, the organic solvent then being stripped of the uranyl nitrate by wateror very dilute nitric acid. Where metallic uranium was desired, the nitrate was then converted to the oxide and then reduced to the metal. See Glasstone, Nuclear Reactor Engineering (New York: D. Van Nostrand Co., 1955), pp. 416-424, and vol. 9, Proceedings of the International Conference on the Peaceful Uses of Atomic Energy (New York: United Nations, 195 6), pp. 453-531.

This stripping technique of the prior art depended for its success on the fact that uranyl nitrate is somewhat more soluble in water than in tributyl phosphate or other organic solvent, in the absence of an excess of nitrate ions. Such excess nitrate, extracted by the tributyl phosphate as a complex with nitric acid along with the uranyl nitrate, was essentially removed by the water in the first stripping stage because nitric acid forms a much weaker complex with tributyl phosphate than uranyl nitrate. However, although the excess of nitrate ions was not present in subsequent stages, the strength of the uranyl nitrate complex with the organic solvent is so high that agreat deal of water was required for complete dissociation and stripping.

4. In the case of some solvents, e.g., tributyl phosphate, it was necessary to substitute a very dilute nitric acid rather than use plain water as the stripping agent to prevent the formation of an emulsion between water and the organic solvent. Strong nitric acid could not be used as this would drive the solution reactions in the wrong direction. f

To obtain reasonable extraction efficiency with such water stripping, it was necessary to use 24 to 30 stripping stages, requiring a columnar structure 25 to 30 feet in ieee height. The very dilute nature ofthe uranium-enriched product necessitated considerable evaporation and addition of nitric acid to restore the proper molarities before the product could be returned to the reactor.

None of these prior art processes involved a high concentration of phosphoric acid in the reactor fuel. On the one hand it was known that tributyl 'phosphate and the other solvents successfully used to extract uranyl nitrate would not extract uranium phosphate complexes. On the other hand, there was no known vsolvent for -th'e extraction of uranium phosphate complexes. Although the tributyl phosphate process indicated that uranyl nitrate could be extracted into that compound, there was no method known in the prior art for vdirectly converting the phosphate complex of a fissionable element to a hexavalent nitrate of such element. No obvious solution to this conversion problem was indicated, as the maximum extraction coefficient obtained with 6.0 M HNOS at a fuel dilution ratio of 16 was only 7.0, as indicated in the publisation by one of the present inventors, Horace R. Baxman, The Extraction of Uranium From Phosphate Solutions, LADC-243l, declassified May 21, 1956 (available through Technical information Service, United States Atomic Energy Commission, P. O. Box 62, Oak Ridge, Tenn), the disclosure of which paper is incorporated herein by reference. The problem presented to the present inventors was one of finding a salting agent which would not only furnish nitrate ions, but would also break down the uranyl phosphate complex without excessive dilution. I

lt is therefore an object of this invention to provide a method for converting uranium phosphate complexes in strong phosphoric acid to uranyl nitrate for subsequent decontamination by extraction into tributyl phosphate.

it is another object of this invention to provide a method for preferentially extracting the uranium in a solution of strong phosphoric acid without excessively diluting such solution. l

A further object of this invention is to provide a method for extracting the uranium in a concentrated phosphoric acid solution from the phosphate and fission products in such acid without precipitating such uranium.

v A still further object of this invention is to provide a method for stripping uranium from a uranium-enriched tributyl phosphate solution without the use of an excessive amount of water or very dilute nitric acid.

Another and further object of this invention is to provide a method for stripping uranium from a uranium enriched tributyl phosphate solution by a method which does not depend on the preferential solubility of uranyl nitrate in water over that in tributyl phosphate in the absence of an excess of nitrate ions.

Another and further object of this invention is to pro vide a continuous process for the removal of fission prod ucts from a solution containing uranium phosphate complexes in concentrated phosphoric acid by an extraction method wherein the product contains the uranium phosphate complex and concentrated phosphoric acid in the same ratio of molar strengths as in the original solution.

Another and further object of this invention is to provide a continuous process for the removal of fission products rfrom a solution of uranium in concentrated phosphoric acid by an extraction method wherein the product of the final stripping and washing stage requires only the evaporation of the nitric acid present and the restoration of any water evaporated with Kthe nitric acid to restore the solution to its original volume containing the original uranium and phosphoric acid molarities. v

The objects of the invention can be achieved by a con# tinuous process of withdrawing a portion of the uraniumenriched phosphoric acid reactor fuel, adding to this fuel solvent, selectivo for uranyl nitrateyextracts suchinitrate f 4,uranium is then restored to its original form and conccn` being drawn off as a rail'inate. The product is boiled to i solvents, salt and stripping agenti, the volume ofj fuel re-` )withdrawn and has the same concentration of .uranium f and ,phosphoric acid. Before returning i the decontaminated. fuel'to thereactor, accumulated losses of uranium stant process are periodically restored. Sinceonly a 25`l'eaC0nS `small volumeof reactorl fuel need; be withdrawn topto-r `a aratus ein loyed,` the vflow of Athe'various-materialsy f riritloand out @fthe kDarts ofthe apparatus, and the vcon- '-Qrm a v3/cake? 69ml-alex Wlthulbutylphosphate accord'. centrationsfand amounts of materials at theivariousistages 35 mg @the reactions: i

i. use involves lthe* decontaminationof the yhi'xm'ogeneous @APRE-1) application of LD. Percival King, entitled Homoge'neous 40- and passes oft inthe organic solventi Since thek tributyl phosphate doesf not have the same y v. fthe disclosure vofwhich assassin o 4 feed lsolution iai-e 0.22` M UO2++,- 2.7 M in l Fe+3 ions, r8.1 M 'in NO3-*and 7.7 N free aci a soluble multivalentinorganic metal nitrate whichwll destroy the phosphate complex ofuranium andr at the same time furnish nitrateions and form a soluble metal f l Phosphatecomplex with a lower dissociation constant,and

"passing the, saltedaqueous. fuel into a countercurrent *f 5= liquid extractionlcoiumn. Therein anlmmscihle organic disclosedm U5. Patent 2.493.265, kissued on January 3.

` 1950, to. E. G. Scheibel, entitled with some nitric acid intosolution and yc'arricsit to they topi of the. column whilethe denser` aqueous solution, being fimmis'cible withr the organic solvent, carries the strong 10 phpha (T BP)r 111 KGIOSBII@ A, a kerosene' fraction rmetal phosphate complex and thecontamination products 'having a' dDS1Y 0f 31201K 0-75 CC-af 25 l in the reactorkfuel'to the bottomy of the column.` yThe 0f 313011989 ITHUIPOISGS at 25, 16S-198 `C.-and an karomatic contentkof about 9.l%, Atration by strippingf'it from the pregnantsolvent with y aqueous :phosphoric acid solution,l 'the organic solvent 15 USedfSOlcly 'aS a dllucut, 1Sr Introduced 'into thebottomof remove ithat nitric acid extractedfand regenerated in thc process.. By suitable selection` ofthe concentrations of flowed from scrub tank 6 atthe turned yto the reactorper unit time is the same as. that the uranyl nitrate inthe tributyl phosphate in the presence fission the reactor from losses in 'the in-' l uranyl nitrate 'feed t()k thel vide adequate. decontamination, ythis volume yis corre` f spondingly added to that required in the reactor and it is not necessary toishut down the reactor. The process is exemplitiedin the sole'drawing attacl'ied 30 hereto and incorporated inthis specification by reference, `i which isy resented-schema icall a `ureshowin 'the Some of these excessimra ons combine Witllsonleof' n 'p' t y g g the yexcess hydrogen ions=from` the phosphoric yacid yto" .of the process for aparticular use of this invention. This H+ IQNO3--: QHNal l HNO3 ;l-TBP:1INO3 TBP jfuelused in the Los Alamos Power Reactor Experiment y n L Thus some nitric acid is extracted with the uranyl nitrati:l

This fuel, as disclosed in the cci-'pending Nuclear rPower Reactor, S.N. 589,837, filed June '6,' 1956,y is` 'incorporated hereinr by refer-y ence, is an aqueous solution of 0.6 M uranyl ion and 7.5 `M H3PO4, with a cold fuel volume of 62 liters at 20 degrees centigrade. The uranium enrichment for this reactor is preferably 93.4%. The reactor is designed to operate at av power level of 2 megawatts.

Referring now to the sole figure, the equipments are indicated by hollow blocks and the flow between equipments by lines with arrowheads to indicate the direction of flow. On each flow line is an arabic numeral indicating the liters per hour of material owing through the line, with the one exception indicated, where the ow is in kilograms per hour.

yFrom the reactor fuel tank 1, there is drawn 0.96 liter of reactor fuel per hour into feed tank 2, such fuel having the composition indicated above and containing the fission `fragments and other contaminants formed by nuclear ssion and other reactions occurring within the reactor.

At the same time 3.06 kilograms per hour of an aqueous solution of ferrie nitrate are fed from salt tank 3 into feed tank 2, said ferrie nitrate being at the concentration of Fe(NO3)3.9H2O dissolved in its own water of hydration at 40-50 degrees centigrade. This results in a dilution of the fuel solution of 2.8 compared with a factor of 16 or more necessary to obtain comparable extractions with nitric acid as the salting agent.

In feed tank 2, approximately one minute of mixing time is allowed to insure thorough reaction between the reactor fuel and the ferrie nitrate salt. In this time the ferrie ions destroy the uranyl phosphate complex and form a ferric phosphate complex, so that thereafter the feed solution contains essentially the ferric phosphate complex, uranyl nitrate, and nitric acid. The concentrations in the 75 .h i'gh solubility: for ythe fission productsy carried by the aqueous feed solution that it does for the uranyl nitrate, most of the lission products remain in the aqueous solu- 45 tion and thus descend through the extraction column 4. However, at the 1.4 organic to aqueous flow ratio used (3.8/2.8) and the 30 volume percent concentration of tributyl phosphate in the kerosene carrier, there is somev excess of the organic solvent and some ission fragments may be picked up. It is the function of the scrub solution'descending from the top through three scrubbing stages to provide an excess of nitrate ion to remove such 'ssion fragments from the tributyl phosphate and carry them to the bottom of the column. Although this scrubbing action by the aluminum nitrate also strips some uranyl nitrate from the tributyl phosphate, such uranyl nitrate is re-extracted by the TBP in the lower stages of the column. The scrub solution-and the uranium-deprived feed solution, carrying oft the fission fragments, descend through ve extraction stages to the bottom of the column, where such rafnate is discharged as a contaminated waste. Owing to volume changes within the column, the discharge rate is slightly less than the 3.3 liter per hour total aqueous input.

The extraction coefficient obtained tion was 18.6, with 99+ from the feed solution. nant solvent are 0.16 M

in this exemplifcapercent of the uraniumextracted The concentrations in the pregin the uranyl nitrate and 0.7-.8

M in HNOa, though both are still present as complexes with tributyl phosphate.

From the top of the extraction column 4, slightly more than 3.8 liters per hour of the pregnant organic solvent are withdrawn and passed to the stripping column 7 just above the second mixing section from the bottom. At the lsame time a 7.5 M solution of H3PO4 from acid tank Poe ai' yThe feed'solutionis 'owed' from feed tankzl at the i rateA of 2.8r liters per hour into the liquid-liquid countery current. extraction column 4, an equipment of the type Extraction Apparatus"y abovethe second mixing section from thebottom: Atj y the same time a SO-'percent byvoiumesoluton ofitributyl C., a viscosity y C., a boiling range yof,

f rcornirionly known as y Gulf BT,`said KeroseneAbeing the extraction column 4 from ksolvent tank 5v at the rate of 3.8 litersy per hour. At the top of the extraction col-y l umn 4, 'an yaqueous scrubgsolution of 1.7 M Al(NO3)3`is rate of 0.5,l'iter per' hour. i o yWhen the l aqueousl feed solution cornes into contact l with `the organic' solvent, themuch greater solubility of of the excess nitrateionsfcausestheTBP to extract the ythe ferrie nitrate yhelpto j drive vthe =first reaction to the yleft yand thek second tothe `right andthusaid inthe extraction.l `'At the same time,

8 is added to the top of the stripping column 7 at the rate of .0.96 liter per hour and a wash solution of Kerosene A from wash tank 9 is added at the bottom of the stripping column at the rate of 0.2 liter per hour. This column is similar in structure to extraction column 4, the only difference being in function, as the top 6 sections are used for stripping and the bottom 2 sections for washing.

The action within stripping column 7 is essentially the destruction of the tributyl phosphate complexes of uranyl nitrate and nitric acid by the concentrated phosphoric acid, the formation of some nitric acid, and the formation of the uranyl phosphate complex with which the process was started.

The reactions involved are:

Since the uranyl phosphate complex is m-uch stronger than the uranyl nitrate complex with tributyl phosphate, the net result of the mixing with excess of phosphoric acid in the absence of other complexing metal ions is the formation of the uranyl phosphate complex. There is no dependence on a small difference in solubilities, as in the case of the stripping of uranyl nitrate by water, because the uranyl phosphate complex is essentially insoluble in tributyl phosphate. Thus it is unnecessary to dilute the phosphoric acid stripping agent and it may be used in the same molar strength as is employed in the reactor itself.

Since uranyl phosphate is insoluble in tributyl phosphate and nitric acid only slightly soluble, the phosphoric acid carries the uranyl phosphate and most nitric acid down the column. The wash solution of kerosene passes up through the descending aqueous solution by -countercurrent operation and removes the traces of tributyl phosphate from the aqueous solution. Together the stripped organic solvent and the wash solution are passed oft from the top of the stripping column 7 as a raflinate at the rate of 4.0 liters per hour. This raffinate may be reworked for solvent regeneration, as indicated in the sole figure. The aqueous part of the mix, consisting of nitric acid, phosphoric acid and uranyl phosphate, is discharged from the bottom of the stripping tank and passed to nitrate removal tank 10 at the rate of 0.96 liter per hour. At thisy time it has concentrations `of essentially 0.6 M UO2++ and 7.5 M 1131304, together with about 1.2 N nitric acid.

In the nitrate removal tank 10 the nitric iacid and some water are removed, by boiling, Iat the rate of about 0.5 liter per hour. The remainder, consisting of an aqueous phosphoric acid solution of the uranyl phosphate complex, is passed to fuel make-up tank 11, where the water removed with the nitric .acid is returned at about 0.5 liter perv hour to dilute the fuel to its original strength of 0.6 M UO2++ and 7.5 M H3PO4. At the same time, Ias a result of periodic samplings, accumulated losses in uranium or phosphoric acid, both in the reactor and in the process, may be made up at intervals. The solution then is being formed at 0.96 liter per hour and is returned at this rate to the reactor to complete the cycle.

The `above-described process has ybeen carried out with reactor fuels having activities of approximately 1.5 euries per liter. Over-al1 losses of uranium were as low as 0.07% in extraction and 0.2% in stripping. Decontamination factors obtained were at least 300 for gross beta activity and 5000 for gross gamma, such factors being diicult to determine because the measured activities of the final product could scarcely be distinguished from background. The two columns employed in the aboveV example are two inches in inside diameter and 50 inches in over-all height. -Only these two columns 4 and 7, the fuel tank 1, nitrate removal tank 10 and fuel make-up tank 11, to-

getherwith interconnecting piping, need be provided with biological shielding. The result is a compact, small-size shielded container for all of the above mentioned equipments, suitable for use with a reactor unit designed for use at remote locations with a minimum of maintenance.

It will be obvious to those skilled in the art that many changes in the above example are possible, depending on the ends to be attained. The process has lbeen successfully applied, for one instance, to an aqueous reactor fuel consisting of a `solution of uranous oxide in a more concentrated phosphoric acid, thus making it applicable to the reactor fuel disclosed in the co-pending application of R. Philip Hammond, entitled Convection Reactor, S.N. 589,836, led lune 6, 1956, the disclosure of which is incorporated herein by reference. The use of the instant process with such fuel requires approximately twice as much dilution of the fuel yas for the 7.5 M phosphoric acid concentration to keep the viscosity of the feed down to a workable level. (This increased dilution would be necessary in any case of an increased acid concentration, regardless of the nature or valence of the tissionable element.) It also requires oxidation of the uranium to .the hexavalent form 'before extraction and reduction to the tetravalent at the end of the process. While the oxidation can be accomplished by the addition of hydrogen peroxide to the reactor fuel, it has been found to occur automatically in the practice of the present invention when the :amount of ferric nitrate (Fe(NO3)39H2O) dissolved in its own water of hydration is added at the rate required to complex all of the phosphate in the fuel, including that in the excess acid. Reduction to the tetravalent form is accomplished by heating with a phosphorous acid, during which the latter is itself converted to the desired phosphoric form. Asmall amount of phosphine gas and elemental phosphorus are also formed in this reconversion, lbut these are easily removed by pumping and screening.

In the above detailed example, the amount of ferric nitrate `added was just thatrequired 'to complex :all of the phosphate in the reactor fuel. The reason for this is apparent from the following table, which shows that a lesser amount of ferric nitrate causes a sharp reduction in the extraction coeiiicient for uranium (E,o (U)) and percent uranium extracted. Since uranium forms a strong complex with phosphate, though not so strong as the ferric phosphate complex, any excess of phosphate ions brought about lby a deficiency of ferric ions would cornplex the uranium' and cause the uranium to remain in the aqueous solution. The cause for the falling olf of extraction efficiency with excess ferric nitrate is an experimental fact, understood at present to be a back-salting of uranyl nitrate out of the tributyl phosphate by nitric acid.

Table l EFFECT OF Fe(NOs)'a-9H2O ON EXTRACTION Oli" URANIUM i Feed Solvent i AIt is obvious that the ratio of organic to laqueous ow rates and the concentration of tributyl phosphate can be varied from the values used in the above detailed ettulrA ple without departing from the spirit 4of" the invention. A rlower ratio of ow rates, organic to aqueous, or a lower concentration of tributyl phosphate at any one ratio would provide higher decontamination because the preferential solubility of uranyl nitrate would saturate the tributyl phosphate, leaving no room for the fission fragment nitrates. However, at the same time, there would be a greater loss of uranium in the aqueous solution, using the equipment and conditions exactly as hereinbefore described. In the case of an increased tributyl phosphate concentration to obtain maximum uranium extraction, a `too large increase in viscosity presents flow problems and makes stripping more difficult. The values used in the 'example aiord a balance -oiering high uranium recovery, reasonable viscosity for stripping and adequate depoison ing, though it is recognized that these values can be shifted A'to obtain greater decontamination without sacrificing eX- ,traction efficiency. This can be achieved, for one instance, by increasing the number of extraction stages with- 4*out altering the flow rates or concentrations in the above example, or by decreasing the aqueous flow rate without changing the organic ow rate and without changing the number of stages or the concentrations. In either case the result is to give the same amount of tributyl phosphate more time to extract the same amount -of available uranyl nitrate. Where it is desirable to provide the maximum removal of all fission products, biologically dangerous and poisonous nuclearly alike, the organic ow rate can be decreased without changing the aqueous ow rate and `more stages can be added..

Although the process of this invention has been applied to uranium enriched in the U-235 isotope, it will 'work as well with normal uranium or any other isotope or `mixture of isotopes of uranium.

The feasibility of the above process as applied to plutonium phosphate, tetravalent, hexavalent or a mixture of such oxidation states, has also been demonstrated. However, tributyl phosphate does not exhibit the same high selectivity for the plutonium nitrates over the fission product'nitrates that it does for uranyl nitrate over the ssion product nitrates. Under the same conditions as were used for extracting uranyl nitrate in the above de- ,tailed description, the extraction coeicient for plutonium is only about 3.0. This means that approximately 20 to 25 times the number of stages are required to obtain 'comparable results.

It is also demonstrable that the other aforementioned organic solvents used to extract uranyl nitrate in the prior art processes will succeed in the process of the present invention, as phosphate complexes are not ex- Ytractable into such solvents. Tributyl phosphate was usedin the embodiment set out above because it possesses a number of advantages, particularly in that it creates no explosive hazard.

In the detailed example above, ferrie nitrate was used as the salting agent because the present inventors discovered that the ferric ion formed the strongest phosphate complex of many metal nitrates tried. Other nitrates such as aluminum nitrate, copper nitrate, calcium nitrate and even nitric acid can be used, but each of these has a much lower extraction coefficient than ferric nitrate. Each thus requires such a higher fuel dilution ratio and so many more extraction stages to obtain comparable results that none achieves the objects of this invention. Furthermore, none of these other nitrates has the ability of the ferrie nitrate to oxidize uranium from the uranous to the uranyl state.

What is claimed is:

1. The process of removing fission products from aqueous solutions containing in concentrated phosphoric acid a phosphate complex of a fissionable element selected from the group consisting of uranium and plutonium comprising the steps of dissociating the phosphate cornplex of the ssionable element and forming a nitrate of the iissionable element, extracting the nitrate of the ssionable element into an organic solvent selected from the class consisting of tributyl phosphate, diethyl ether, methyl isobutyl ketone, amyl alcohol and amyl acetate, and stripping the fissionable element from said organic solvent with concentrated phosphoric acid of the same molar strength as in said aqueous solution.

2. A process for removing fission products from aqueous solutions containing a uranyl phosphate complex in concentrated phosphoric acid comprising the steps of dissociating the phosphate complex of uranium and forming uranyl nitrate, extracting the uranyl nitrate into an organic solvent selected from the class consisting of tributyl phosphate, diethyl ether, methyl isobutyl ketone, amyl alcohol and amyl acetate, and stripping the uranium from said organic solvent with concentrated phosphoric acid of the same molar strength as in said aqueous solution.

3. A continuous process for removing fission products from aqueous solutions containing in concentrated phosphoric acid a phosphate complex of a fissionable element selected from the group consisting of uranium and plutonium, comprising the steps of adding ferrie nitrate to said aqueous solution, extracting the resulting nitrate of the fissionable element into a solvent selected from the class consisting of tributyl phosphate, diethyl ether, methyl isobutyl ketone, amyl alcohol and amyl acetate, stripping the fissionable element from said solvent with concentrated phosphoric acid, evaporating from the product the nitric acid extracted with the ssionable element, and adding the necessary water to restore the product to the ,molar strengths of fissionable element and phosphoric acid lasinthe original solution.

4. A continuous process for removing fission products from aqueous solutions such as reactor fuels containing a uranyl phosphate complex in concentrated phosphoric acid, comprising the steps of adding to said solution an amount of ferrie nitrate solution having a number of ferrie ions equal to the total number of phosphate ions in said solution and having a volume to dilute said solution by a ratio not exceeding three for a 7.5 molarity of said acid in said solution, extracting said uranium in the resulting form of uranyl nitrate into a tributyl phosphate solution, stripping the uranium from said tributyl phosphate solution with concentrated phosphoric acid of the same molar strength as in said aqueous solution, evaporating any nitric acid present, and restoring the water evaporated with the nitric acid.

5. A continuous process for removing fission products from aqueous solutions containing a uranous phosphate complex in concentrated phosphoric acid, comprising the steps of adding to said solution an amount of ferrie nitrate solution containing a number of ferrie ions equal to the total number of phosphate ions in said solution and having such a volume as to dilute said solution by a ratio not exceeding six for a strength of said acid., extracting said uranium in 'the resulting form of uranyl nitrate into a tributyl phosphate solution, stripping said uranium from said tributyl phosphate solution with concentrated phosphoric acid, evaporating any nitric acid in said phosphoric acid, heating said phosphoric acid with phosphorus acid, and removing any resulting phosphine gas and elemental phosphorus.

6. A process for removing fission products from an aqueous solution containing a uranyl phosphate complex in concentrated phosphoric acid, comprising the steps of adding to said solution a ferrie nitrate solution having a number of ferric ions equal to the total number of phosphate ions and a volume to dilute said solution by a ratio not exceeding three for a 7.5 M strength of said acid or 6.0 for 95% acid, extracting the resulting uranyl nitrate into a solvent selected from the class consisting of tributyl phosphate, diethyl ether, methyl isobutyl ketone, amyl alcohol and amyl acetate, stripping the uranyl ion from said solvent in the form of the original uranyl phosphate complex with concentrated phosphoric acid, evaporating any nitric acid present, and adding phosphoric acid in the concentration necessary to give the molarities of uranyl phosphate and phosphoric acid in said original aqueous solution.

References Cited in the le of this patent UNITED STATES PATENTS Hagemann et al. Apr. 24, 1956 OTHER REFERENCES Smellie et al.: AECD-3906, July 16, 1947, pp. 39, 51.

U.S. Atomic Energy Commission, K706, by T. W. V 

1. THE PROCESS OF REMOVING FISSION PRODUCTS FROM AQUEOUS SOLUTIONS CONTAINING IN CONCENTRATED PHOSPHORIC ACID A PHOSPHATE COMPLEX OF A FISSINABLE ELEMENT SELECTED FROM THE GROUP CONSISTING OF URANIUM AND PLUTONIUM COMPRISING THE STEPS OF DISSOCIATING THE PHOSPHATE COMPLEX OF THE FISSIONALBE ELEMENT AND FORMING A NITRATE OF THE FISSIONABLE ELEMENT, EXTRACTING THE NITRATE OF THE FISSIONABLE ELEMENT INTO AN ORGANIC SOLVENT SELECTED FROM THE CLASS CONSISTING OF TRIBUTYL PHOSPHATE, DIETHYL ETHER, METHYL ISOBUTYL KETONE, AMYL ALCOHOL AND AMYL ACETATE, AND STRIPPING THE FISSIONABLE ELEMENT FROM SAID ORGANIC SOLVENT WITH CONCENTRATED PHOSPHORIC ACID OF THE SAME MOLAR STRENGTH AS IN SAID AQUEOUS SOLUTION. 